This invention relates to an X-ray microscope and more particularly to a new type of X-ray microscope using an imaging technique combined with production and control of Moire patterns by X-ray diffraction from crystals. Hereafter referred to as the X-ray Moire microscope (XMM)
X-ray microscopes are described in "X-ray Microscopes", by Malcolm R. Howells, Janos Kirz and David Sayre, Scientific American, Feb. 1991, Vol. 264, pp. 8-94. As stated in this publication, the development of X-ray crystallography early in the twentieth century yielded accurate images of matter at atomic resolution. Subsequently electron microscopes have been developed and provide direct views or viruses and minute surface structures. Another type of microscope utilizing X-rays rather than light or electrons, provides a different way of examining tiny details, and considerably improves on the resolution of optical microscopes. They can also be used to map the distribution of certain chemical elements, form pictures in extremely short times, and have the potential for special capabilities such as 3-dimensional imaging. X-ray microscopy differs from conventional electron microscopy in that specimens can be kept in air and in water, whereby biological samples can be studied under conditions similar to their natural state.
As further described in the above article, imaging X-ray microscopes use focusing optics to form an image magnified a few hundred times, which can then be recorded by a detector of modest resolution. The principle benefit of imaging X-ray microscopes is that the entire sample is illuminated and imaged at once, which permits rapid picture taking thereby combatting blurred images resulting from motion and minimizing radiation damage in biological samples.
FIGS. 1 and 2 from the above article show examples of an imaging X-ray microscope and a scanning X-ray microscope. In FIG. 1, an X-ray beam 100 from an X-ray source (not shown) passes through a condenser zone plate 102 which focuses the beam on a sample 104 held in a sample holder 106. A micro-zone plate 108 magnifies images of the sample on a detector 110. The image field is indicated at 112. Fresnel zone plates serve as condenser 102 and objective 108 X-ray lenses. In the scanning X-ray microscope shown in FIG. 2, X-ray beam 114 from an X-ray source passes through a source pinhole 116 onto a zone plate 118 and then through an aperture 120 onto the sample 122 held in a sample holder with X-Y raster scan 124 and then onto an X-ray counter 126. The focused X-ray beam scans back and forth, top to bottom across the sample. The rays that penetrate at each point are measured using a proportional X-ray counter.
U.S. Pat. No. 4,870,674 shows an X-ray microscope of the type wherein the object is illuminated at least partially coherently by a condenser with quasi-monochromatic X-ray radiation and is imaged enlarged in the image plane by a high-resolution X-ray objective. An element which imparts a phase shift to a preselected order of diffraction of the radiation is arranged in the Fourier plane of the X-ray objective to obtain the highest possible image contrast.
U.S. Pat. No. 5,027,377 describes an X-ray microscope or telescope having a connected collection of Bragg reflecting planes comprised of either a bent crystal or a synthetic multi-layer structure disposed on and adjacent to a locus determined by a spherical surface, for producing sharp chromatic images of magnification, which may be greater than or less than unity, from radiation within X-ray band widths propagated from X-ray emitting objects.
U.S. Pat. No. 5,044,001 describes investigating materials by the use of X-rays including a chamber having a wall with an aperture in which is mounted a support substrate composed of a material substantially transparent to X-rays, a first surface on the substrate facing the interior of the chamber and a second surface facing outside the chamber, a metal foil on the first surface having a thickness of less than about 0.1 .mu.m exposed to the interior of the chamber. A beam of electrons is focused within the chamber on the metal foil to a beam diameter of less than about 1,000 .ANG. incident on the metal foil. The specimen outside the chamber is positioned adjacent to the second surface of the substrate, and at least one X-ray detector is positioned to detect X-rays leaving the specimen. The X-ray detector is an energy dispersive type capable of selecting and recording a narrow range of peak energy and energies close to peak energy.
U.S. Pat. No. 5,016,265 describes a variable magnification variable dispersion glancing incidence X-ray spectroscopic telescope capable of multiple high spatial high revolution imaging at precise spectral lines of solar and stellar X-ray and extreme ultraviolet radiation sources, wherein the spectrum bandpass is readily selectable from a plurality of multi-layer diffraction grating mirrors aft of the primary focus of the primary glancing mirrors on a rotatable carrier, and the magnification and field of view are selectable from a plurality of such carriers, the image being resolved onto one or more X-ray detectors. X-rays of the selected wavelength are reflected and diffracted to produce an overlapping array of images to a detector at the second focus of elliptical diffraction mirrors. Each image corresponds to the emission from the plasma in a single spectral line. The different diffraction grating mirrors on each rotating carrier have the same surface contour, but are coated with multilayer coatings of different multilayer compositions or 2D parameter.
U.S. Pat. No. 3,439,164 describes a method of obtaining X-ray interference patterns using two parallel perfect crystals of the same thickness which exhibit the Borrmann effect, wherein the crystals are oriented so that a monochromatic X-ray beam incident on the first crystal is simultaneously diffracted from two independent sets of planes in the crystal, the two forward diffracted beams are parallel and are directed at a second highly perfect relatively thick crystal whereby four forward diffracted rays are transmitted from the second crystal, two of the four forward diffracted rays transmitted by the second crystal coincide with each other and the phase of the one of the rays incident on the second crystal is varied relative to the other to vary the interference pattern formed by the coincident rays.
Conditions for the formation and observation of X-ray Moire patterns in crystalline systems are discussed in "Main Crystallographic Situations for the Formation of X-ray Moire Patterns", by P. A. Bezirganyan, S. E. Bezirganyan, and A. 0. Aboyan, Phys. Stat. Sol. (a) 126, 41 (1991); "Use of the Ewald Sphere in Aligning Crystal Pairs to Produce X-ray Moire Fringes", by Jay Bradley and A. R. Lang, Acta Cryst. (1968) A 24, 246; and "Dynamic Scattering of X-rays in Crystals" by G. Pinsker, Springer-Verlag, Berlin, 1978.
The development of X-ray microscopes has faced a number of technology problems and up to the present time it cannot be claimed that all of these problems have been solved.
The established approaches can be placed in one of several categories including scanning microscopes, imaging X-ray microscopes, image converting microscopes, and X-ray holography. Scanning X-ray microscopes, like scanning electron microscopes, use a small probe beam of X-rays to produce a signal. which is recorded as either the probe is scanned across the sample or as the sample is scanned through the beam. The small size of the probe beam can be produced either by a focusing element or by a simple pinhole. Imaging X-ray microscopes are more analogous to conventional optical microscopes in that the sample is uniformly illuminated and imaged onto an area detector or film by a magnifying optical element. Image converting X-ray microscopes involve a simple contact image of the sample being recorded onto a photoresist, with the actual magnification performed by electron micrography of the developed resist. X-ray holography depends upon recording the patter of interference between radiation scattered by the sample and a coherent reference source of X-rays.
The technological limitations for these traditional approaches toward X-ray microscopy involve the difficulties of producing suitable optical elements, detectors, photoresists and sources for the X-ray wavelengths employed. X-rays interact weakly with matter, which is an advantage for many applications, but is a severe disadvantage for producing optical elements. Even producing a pinhole, as required for some scanning microscopes, is difficult in that the material around the pinhole must be thick enough to stop the unwanted portions of the illuminating X-ray beam. Both imaging X-ray microscopes and X-ray holography require position sensitive area detectors whose position resolution is ultimately limited by the volume of material required to completely contain the energy deposited by the X-rays upon detection. Production of coherent beams of X-rays tends to limit the total signal even from the brightest X-ray sources available. In addition to holography, X-ray coherence is required for optimal performance of Fresnel zone plates which are the most successful type of X-ray focussing element demonstrated to date. All of the above problems have been addressed with the most success in the soft X-ray regime, i.e., 100 eV-1000 eV where they are typically less severe than at higher photon energies.
The concept of forming images through mathematical transformation is related to Hadamard Transform Imaging used in X-ray astronomy. A Hadamard transform is analogous to a Fourier transform where the former uses square-wave modulation and the latter uses sine-waves. Computed X-ray tomography is also an is imaging technique that depends upon a mathematical transformation of recorded data (G. K. Sinner, Scientific American 260 (8) (1988) 84; P. J. Treado and M. D. Morris, Anal. Chem. 61 (1989) 723A).
X-ray Moire patterns produced by X-ray diffraction from two or more crystals have been observed on a macroscopic scale. These patterns are used in studies of crystal defects and are observed in X-ray interferometry (J. Bradley and A. R. Lang, Acta Cryst. A 24 (1964) 246., U. Bonse and M. Hart, Z. Phys. 188 (1965) 154).